Vacuum metal casting apparatus

ABSTRACT

A metal casting apparatus especially adapted for the rapid melting and casting of small charges of metal, utilizing a metalheating crucible in communication with a mold contained in a heated enclosure. The metal charge within the crucible is rapidly heated to melt the same to bring the molten metal to optimum casting temperature, whereupon the molten metal in the crucible is subjected to sufficient pressure differential to force the molten metal through a small opening in the crucible into the mold.

United States Patent 1191 Daniel et al.

[ Jan. 29, 1974 FOREIGN PATENTS OR APPLICATIONS 1 1 VACUUM METAL CASTINGAPPARATUS Great Britain...... 164/65 GreatBritain..1...,................, 164/80 ABSTRACT A metal castingapparatus especially adapted for the rapid melting and casting of smallcharges of metal, utilizing a metal Primary Examiner-R. Spencer AnnearAttorney, Agent, or FirmDonnelly, Maky, Renner & Otto heating cruciblein communication Inventors: Arnold Daniel, 2219 Devonshire Dr.; JosephBurdette Richey, I], 2160 Demington Dr., both of Cleveland Heights, Ohio44106 [22] Filed: Oct. 26, 1972 [21] Appl. No.: 301,158

Related U.S. Application Data [62] Division of Ser. No. 92,583, Nov. 25,1970, Pat. No.

with a mold contained in a heated enclosure. The metal charge Within thecrucible is rapidly heated to melt the same to bring the molten metal tooptimum casting temperature, whereupon the molten metal in 56 6 fi5lw5 22 M w. M 1 2 2 4 5 M U 6 3/ .1. 64 s 6 0 "2 5 "6 n m 4 M 6 A 1 m6 Tn 1 ca .r n "a e m .L hf. C 10 u min U IF MM UN 5 55 22 Claims, 5 DrawingFigures the crucible is subjected to sufficient pressure differential toforce the molten metal through a small opening in the crucible into themold.

164/62 UX 164/62 UX [56] References Cited UNITED STATES PATENTS1,070,500 8/1913 Mackenzie.....................

1,644,543 10/1927 Richmond......................

VACUUM METAL CASTING APPARATUS CROSS-REFERENCE TO RELATED APPLICATIONSThis application is a division of our copending United Statesapplication Ser. No. 92,583, filed Nov. 25, 1970, now U.S. Pat. No.3,712,364, granted Jan. 23, 1973.

BACKGROUND OF THE INVENTION Investment casting procedures have long beenemployed in the production of investment castings of pre cious metalsfor use in jewelry, dental inlays and bridges, and the like. The presentsystem is particularly advantageous when very precise control of thecasting operation is required in order that the casting may be of exactpredetermined dimensions and alloy composition.

In the conventional production of dental alloy castings, an investmentmold is prepared about a wax model or pattern and the mold is preheatedto volatilize the wax and to bring the mold up to the desired castingtemperature. The achievement and maintenance of such temperature duringthe casting operation is of considerable importance inasmuch as both themold and the casting tend to shrink upon cooling and in order to obtaina finished casting of exact predetermined dimensions it is essentialthat both the molten metal and the mold be at proper temperatures whenthe molten metal is introduced into such mold. In a typical operation,the mold has thus been preheated and then transferred to a centrifugalcasting machine. The gold alloy charge is placed in the machine andheated by means of an oxyacetylene torch to melt the charge, whereuponthe machine is activated and the molten metal forced into the mold underthe influence of centrifugal force. Under such circumstances, it isobviously not possible to ensure that the mold is at the precise optimumtemperature at the moment the molten metal enters the same and it isalso not possible to ensure that the molten charge itself is at anoptimum temperature. Furthermore, the alloy composition may be somewhatmodified during the heating operation and the charge may be contaminatedby atmospheric gases and the gases produced by employment of such torch.An appreciable excess of the alloy must be provided requiring subsequentcutting and grinding away of the metal sprue produced, and the entireoperation requires such a degree of skill and experience that manydental offices prefer to send such work to specialist laboratoriesrather than to perform the work themselves. This results in considerabledelay and increased expense, particularly when several castings must beproduced until one is achieved which is found to be entirelysatisfactory for the intended purpose.

It is accordingly an object of the present invention to provide aself-contained unit wherein the mold need not be removed from the heatbath prior to or during performance of the casting operation.

It is also an object to provide a metal casting system and apparatus forperforming the same of small size which may be operated from theordinary 1 volt AC source available in all dental offices and which willincorporate a control system to ensure the automatic production of highquality castings without the requirement of any special skills on thepart of the operator.

A further object is to provide a metal casting system wherein both themolten metal and the mold are brought to the optimum temperatures andthe casting operation is performed very rapidly in a manner to keep themolten metal charge uncontaminated and to maintain the proportions ofany alloy elements unchanged.

Other objects of the invention will appear as the description proceeds.

To the accomplishment of the foregoing and related ends, the invention,then, comprises the features hereinafter fully described, the followingdescription and the annexed drawings setting forth in detail certain i1-lustrative embodiments of the invention, these being indicative,however, of but a few of the various ways in which the principles of theinvention may be employed.

DESCRIPTION OF THE DRAWING In said annexed drawing FIG. 1 is anisometric view of a novel metal casting apparatus, including associatedcontrol means, for practicing the method of the present invention;

FIG. 2 is an enlarged vertical longitudinal section taken on the line2-2 of FIG. 2, but omitting the control panel;

FIG. 3 is a wiring diagram illustrating suitable control means for theoperation of such apparatus;

FIG. 4 is an enlarged exploded isometric detail view of the meltingcrucible assembly of such apparatus; and

FIG. 5 is a wiring diagram of an optional modification of the metalheating means.

DESCRIPTION OF THE INVENTION In accordance with the invention,investment casting apparatus is provided comprising an insulatedenclosure or muffle having heating means for heating the interiorthereof to a high temperature. A metal heating crucible is mounted inthe upper portion of such enclosure and means is provided to support amold directly beneath such crucible in clamping engagement therewith sothat molten metal produced in the crucible may be directly dischargedinto the mold therebeneath. Suction means is applied to the mold toassist in drawing the molten metal charge from the crucible into themold cavity, and electrical control means ensure that the variousoperations are carried out at the proper times and at the propertemperatures. The melting crucible may be enclosed within an inertatmosphere to avoid any possibility of contamination of the moltencharge.

Now referring more particularly to said drawing and especially FIGS. 1and 2 thereof, the embodiment of the invention there illustratedcomprises a furnace or oven 1 desirably supported on legs such as 2 andhaving an outer sheet aluminum shell or casing 3. Such casing is linedwith a thick wall of refractory insulating material 4 which encloses arefractory ceramic inner liner 5. Nichrome heating elements 6 areinterposed between such outer liner 4 and inner liner 5 in order toraise the interior of the furnace to a high temperature. The front ofthe furnace is closed by an insulated hinged gate or door 7 normallyheld in closed position by means of counterweight 8.

A control panel 9 is carried by brackets spacing the same from the endof the furnace, and a removable hood 10 is mounted on the top of thefurnace. Such hood may be connected, if desired, to a source of inertgas such as cylinder 11 by means of flexible tubing 12,

admission of such gas (e.g. helium, nitrogen, or argon) to the hoodbeing controlled by means of hand operated valve 13.

Hood is adapted to cover an opening in the upper side of the furnacewithin which a metal heating crucible is received and mounted. Suchcrucible is a vertically split graphite crucible comprising halfportions 14 and 15 (see also FIG. 4) separated by a layer of inertrefractory material such as asbestos 16 which may be provided with acentrally disposed gap in the lower edge thereof to serve as a sprue forthe crucible; the two halves of the crucible are thus electricallyinsulated from each other. In the preferred embodiment illustrated indetail in FIG. 4 the sprue 17 is, however, provided in one crucible halfonly, with the inner surfaces of the other half and such asbestosdivider being inclined theretoward. Such sprue may, for example, beapproximately 0.04 mm. diameter. Accordingly, when the crucible blocks14 and 15 are assembled with the interposed insulating layer 16 theydefine a crucible cavity the upper end of which projects above the uppersurface of furnace 1 and the lower end of which projects into theinterior of furnace l. The graphite blocks 14 and 15 are respectivelybacked by heavy sheet aliminum or copper contact plates 19 and 20 to theupper edges of which electric power cables 21 and 22 are connected.Layers of electrical insulation 23 and 24 laterally enclose the crucibleand contact plates so that the two halves of the crucible are not onlyelectrically isolated from each other but also from the supporting upperfurnace wall.

An optional, but ordinarily highly preferred device is also providedwithin hood 10 to bear downwardly upon a metal charge to be meltedwithin the crucible and thereby ensure better contact between the twohalf sections of such crucible and the charge which bridgestherebetween. Such device may comprise a bracket 25 mounted on the uppersurface of the furnace and carrying a vertically disposed plunger 26directly axially reciprocable above the sprue of the graphite meltingcrucible.

A compression spring 27 serves normally to maintain such plunger indepressed condition so that the graphite rod 28 coaxially securedthereto may thus bear firmly downwardly upon the charge in the crucible.Plunger 26 and rod 28 may be temporarily manually elevated when it isdesired to place a new charge within the crucible by depressing leverhandle 29.

A circular metal, e.g. stainless steel, jig or platform 30 is supportedwithin the furnace chamber or muffle 31 on the upper end of verticallyreciprocable metal tube 32. The upper end of such tube opens to acentrally disposed orifice in the. upper surface of platform 30 and thelower end of such tube connects through an elbow 33 of stainless steeltubing to vacuum hose 34. A manually rotatable nut 35 is threaded on thelower projecting end portion of metal tube 32 and held against axialmovement by upper washer 36 interposed between such nut and theunderside of the furnace and by lower washer 37 interposed between suchnut and the centrally apertured spider 38 affixed to the underside ofthe furnace and projecting downwardly therefrom. Accordingly, when nut35 is thus manually rotated, jig or platform 30 will be correspondinglyelevated or lowered within the furnace chamber.

The remaining elements of the assembly may best be understood from thedescription of the operation of the metal casting system set forthbelow.

OPERATION The operation of this new metal casting system will first begenerally described, and then more specifically with reference to theelectric control system.

The usual wax pattern or model is prepared and embedded in a suitableporous plaster mold 39 enclosed in the usual tubular metal casting ring40. Such mold may desirably initially be placed within furnace chamber31 laterally of platform 30 as shown at 40' for preheating prior toperformance of the casting operation. Such preheating serves tovolatilize and drive off the wax pattern, the fumes being vented fromchamber 31 through vents V, and to heat the mold 39 to proper castingtemperature. The door to the furnace will then be opened and theoperator will lift the mold onto platform 30 where it is supported onannular asbestos sealing ring 41 which serves thus to space theunderside of the mold slightly from the upper surface of platform 30,thereby permitting suction subsequently to be applied to the entireundersurface of the mold through metal tube 32. In some cases, if themold is not sufficiently porous, it may be desirable to provide a verysmall vent passage 42 leading from the mold cavity 43 to a layer ofporous asbestos 44 extending to the lower surface of mold 39 and therebypermitting suction (e.g. 28-29 inches of mercury) to be applied moredirectly to such mold cavity. The upper surface of the mold is crateredat 45 to provide a funnel shaped cavity terminating in a small sprue 46leading to the mold cavity 43. A centrally apertured sheet metal cover47 fits over the top of the mold and asbestos sealing ring 48, and asmaller asbestos sealing ring 49 is interposed between such cover andthe underside of the crucible so that the sprue 17 of the cruciblecommunicates directly with a closed vacuum system.

A crucible assembly is clamped firmly in place by means of large setscrew 50 which may be insulated from contact plate 19, or which maymerely be grounded by grounding of outer aluminum casing 3 of thefurnace). The hand nut 35 will now be rotated to elevate platform 30 toraise the mold and clamp cover 47 firmly against sealing ring 49interposed between the same and the underside of the split crucible.

Cover 10 is now lifted, handle 29 is depressed to raise rod 28, and themeasured solid metal charge 51 is placed in the crucible. Handle 29 isthereupon released so that rod 28 may firmly press such chargedownwardly against the respective crucible half sections. Heatingcurrent may now be directed to the crucible sections through leads 21,22 for a sufficient period of time (e.g. 10 seconds) to melt the metalcharge. The plunger assembly 26, 27, 28 will ordinarily be sodimensioned and proportioned that, even when the crucible is completelyempty, the lower end of rod 28 will not extend entirely to the bottom ofthe crucible, it being desired only that the lower end of such rodshould bear upon the solid metal charge with sufficient force to ensureadequate electrical contact between such charge and the respectivecrucible sections. When the charge has melted and been brought tooptimum temperature (as determined by thermocouple or infrared detectorsensing means described in more detail in connection with the discussionof the wiring diagram,

below) suction is applied to tubing 34 and therefore through elbow 33,tubular support 32, porous mold 39, mold cavity 43, and sprue 46 to thecrucible sprue to overcome the surface tension of the molten metal andrapidly draw the latter downwardly into the mold cavity. Platform 30 maythereupon be lowered, door 7 opened and the mold removed from chamber31. Alternatively, if it is desired to cool the mold more slowly inorder better to control the solidification phenomena which affect thegrain structure of the casting, the mold may be left within such chamberfor an extended period of time while the chamber is very graduallycooled.

It will be apparent from the foregoing that the molten metal may thus bevery rapidly drawn into the mold cavity immediately upon attainment ofoptimum temperature and with minimal opportunity to absorb anyimpurities. As above explained, an inert gas may be admitted to cover orhood 10 from cylinder 11 prior to performance of the melting operationand, if desired, such inert gas may even be admitted under sufficientpressure substantially to purge the hood and crucible prior to elevationof the mold into clamping relationship with the underside of thecrucible. The inert gas pressure within the hood may thereupon assist inejecting the molten metal from the crucible into the evacuated moldcavity if the hood is tightly clamped down and the crucible isadequately sealed at the top of the furnace.

Now referring more particularly to FIG. 3 of the drawing, the operationof the metal casting system of this invention will be described ingreater detail, with particular reference to the electrical controlsystem. After the investment rings 40 and 40' with their enclosed moldshave been placed within furnace chamber 31, and door 7 closed, lineswitch 52 will be manually closed to admit 100 volt AC heating currentfrom main line 53, 54 to lines 55, 56 connected with the nichromeheating elements 6 in furnace 1. Line 56 is connected through a standardtemperature control unit 57 which may be preset to disconnect such linewhen a desired temperature has been attained within the furnace.Temperature sensing means in the form of the thermocouple 58 may beprovided in the furnace and connected with such temperature control unit57 to activate the latter. A neon pilot light 59 signals that power ison.

Hand screw 35 isrotated to elevate platform 30 to clamp the mold inplace beneath the split crucible, and the metal charge 51 is placedwithin such crucible and downwardly biased rod 28 is caused to pressthereon. Hood 10 is closed, clamped, and inert gas is admitted theretoif desired.

When thermocouple 58 signals that the predetermined temperature has beenattained in furnace chamber 31, the temperature control unit 57automatically cuts off or limits further power to the nichrome heatingelements 6 to maintain the constant preset temperature. After sufficienttime is allowed for the mold to reach optimum temperature, the operatormay thereupon initiate the melting cycle by depressing the reset switch60 on crucible temperature controller 61. This energizes the coil ofcontactor 62 which in turn applies line voltage to the primary oftransformer 63, causing very high current to flow through the secondaryof such transformer, the two graphite mold sections 14 and 15, and themetal charge 51 bridging the latter. When the predetermined meltingoptimum temperature of the molten metal produced has been sensed andsignaled by thermocouple 64 embedded in the crucible but electricallyinsulated therefrom or by infrared detector D located above thecrucible, the temperature control unit 61 deenergizes the coil ofcontactor 62 and energizes the coil of time delay relay 65 which in turnenergizes the coil of normally closed solenoid valve 66 to open thelatter and connect suction line 34 to vacuum for a length of timedetermined by the time delay relay. This immediately operates as abovedescribed to draw the molten metal through the crucible sprue into theevacuated mold cavity 43. After a preset time determined by time-delayrelay 65, valve 66 automatically recloses, cutting off the vacuumconnection to the mold.

During application of the vacuum, the pressure differential is effectiveto overcome the surface tension of the molten metal and force the latterthrough the small crucible sprue.

It will be seen from the foregoing that a method of discharging moltenmetal from the crucible has been provided wherein the outlet or sprueleading therefrom is of sufficiently small size that the surface tensionof the molten metal normally serves to prevent discharge therethrough bygravity, the suction which is then applied at the proper momentaffording a pressure differential sufficient to overcome such effect ofsurface tension and cause the molten metal to flow through the sprue.The crucible should, of course, be of material not wettable by suchmolten metal.

The investment may be prepared in generally wellknown manner for theproduction of cast dental inlays, onlays, crowns, bridges, andframeworks for removable appliances, utilizing dental casting goldalloys in accordance with American Dental Association specification No.5 (Federation Dentaire Internationale Specification No. 7 for DentalCasting Gold Alloy) Approved April, 1965; Effective April 1, 1966.

The wax pattern is prepared and a 12 gauge (Brown & Sharpe) sprue pin isinserted in the bulkiest part of the pattern and a conical sprue formeris affixed to the other end of such pin.

Normally, the extent of such pin from the pattern to the sprue formershould not exceed three-eighth inch. The assembly is carefully washedand allowed to dry.

A cylindrical brass or stainless steel casting ring is lined with acircularized strip of asbestos, the ends of which overlap slightly. Suchring may be 1 /2 inch in length and 1 A; inch in diameter, for example,and the asbestos liner should be about one-eighth inch shorter than thering, at each end.

The investment plaster (usually a gypsum composition) such as KerrLuster Cast or Cristobolite is mixed with water (e.g. 30 to 34 parts ofwater to grams of powder), the plaster being sifted into the water,until a creamy consistency is obtained. The pattern is painted with themixture, usually after first lightly painting it with a wetting agent,and inverted and placed within the ring which is then filled with thesame mix.

After being allowed to set for about one hour, the burnout operation maybe commenced. The sprue former and sprue pin are removed and the castingring and investment may now be placed in the furnace with the sprue holein the down position. When the mold has gradually been brought toapproximately 500F it is turned with the sprue upward and brought tocasting temperature (l,292 F in reference to Cristobolite). It shoulddesirably be allowed to heat soak for about 20 minutes at thistemperature prior to casting.

The crucible is charged with ingots of gold alloy (about 2 to 4pennyweight for inlays and crowns, and about 20 to 30 pennyweight forlarge partial framework castings) and brought to fusion temperature,this ordinarily being nearer the upper (liquidus) than the lower(solidus) limit of the melting range.

The mold is clamped and the vacuum applied as described above. Thevacuum should be high, e.g. 28-29 inches of mercury, if possible, (l2-l3psi) and the diameter of the crucible sprue opening may be 16 gauge(Brown & Sharpe) or less to prevent premature dribbling.

Now referring more specifically to FIG. 5 of the drawing, the optionalform of hold-down there illustrated may comprise a graphite electrode 67corresponding to rod 28 in function but also being electricallyconnected to transformer 63. It must accordingly be electricallyinsulated as at 68 in its support, and a portion of the heating currentwill flow between the electrode and the crucible sections.

The temperature sensing thermocouple 64 (FIGS. 3

and 4) may have its leads extending through an elongated ceramicinsulating member 69 inserted in one of the graphite crucible blockswith the thermocouple itself embedded in refractory ceramic insulatingcement adjacent the metal fusing Zonev Insulating pads 70 and 71 may beprovided on the crucible projections such as 72 and 73 to ensure thatthe crucible is insulated from the furnace under the upward pressure ofthreaded tube 32 and also to provide a gas tight seal around thecrucible.

The infrared radiation detector D referred to above as a preferredalternative to thermocouple 64 is a commercially available unitsemi-diagrammatically shown in FIG.. 2 as mounted in an extension ofbracket 25. Such detector (preferably the silicon type) is clamped at anangle to focus upon the charge 51 in the bottom of the crucible andthereby is enabled to sense the temperature of such charge by responseto the infrared radiation of the metal (infrared radiation pyrometry).The leads from detector D thus connect with temperature controller 61instead'of the leads shown coming from thermocouple 64 in FIG. 3.

The apparatus and method of the present invention accordingly provide ametal casting system adapted to ensure the integrity of the resultingcasting as to composition, physicals, and dimensions to a degree notheretofore readily attainable. Such apparatus is not unduly expensive,and the mode of operation is such that it may be carried out without thebenefit of exceptional skill and training.

We, therefore, particularly point out and distinctly claim as ourinvention:

1. Metal casting apparatus comprising an insulated enclosure, a moldwithin said enclosure, heating means for heating the interior of saidenclosure to heat said mold to optimum casting temperature, ametal-heating crucible above said mold having an opening incommunication with said mold within said enclosure, additional heatingmeans for rapidly heating a metal charge within said crucible to meltthe same and bring the molten metal to optimum casting temperaturewithin said crucible, said opening in said crucible being too small topermit ready passage of such molten metal therethrough, means forsensing the temperature of the molten metal in said crucible, and meansfor subjecting such molten metal in said crucible to sufficient pressuredifferential to force such molten metal through said opening into saidmold after the molten metal has reached such optimum castingtemperature.

2. The apparatus of claim 1 wherein said additional heating means forrapidly heating the metal charge in said crucible comprises electricalcontact plates connected to said crucible for passage of electriccurrent through said crucible and through the metal charge to causeresistance heating of the latter.

3. The apparatus of claim 1 wherein said crucible is vertically split toprovide split portions of electrically conductive material, withinsulating material separating said split portions, and said additionalheating means for rapidly heating the metal charge in said cruciblecomprises means for passing an electric heating current between saidsplit portions through the metal charge contained therein to melt suchmetal charge.

4. The apparatus of claim 3 further comprising means for pressing suchmetal charge against said crucible split portions to enhance passage ofsuch electrical current through such metal charge rapidly to heat thesame.

5. The apparatus of claim 3 further comprising a holddown member mountedto bear downwardly upon the metal charge in said crucible to insure goodelectrical contact of such metal charge with both of said crucible splitportions to enhance passage of such electric current through such metalcharge rapidly to heat the same.

6. The apparatus of claim 5 wherein said hold-down member is alsoelectrically conductive and is connected to an electric current sourcefor passage of current between said hold-down member and said cruciblesplit portions through such metal charge.

7. The apparatus of claim 5 wherein said hold-down member is resilientlymounted for downward pressure upon such metal charge, and stop means areprovided for limiting downward movement of said hold-down member toprevent direct contact of said hold-down member with said crucible splitportions.

8. The apparatus of claim 1 wherein said mold is a suction mold, andsaid means for applying such pressure differential to force such moltenmetal through said opening in said crucible into said mold comprises asuction pump for applying suction to said mold to draw such molten metalfrom said crucible into said mold.

9. The apparatus of claim 8 further comprising clamping means forclamping said mold in sealing engagement with the underside of saidcrucible thereby to permit application of such suction to said cruciblethrough said mold.

10. The apparatus of claim 1 wherein the surface tension of such moltenmetal is sufficient to prevent passage of such molten metal through saidcrucible opening under the influence of gravity, and such pressuredifferential is effective to overcome such surface tension when applied.

11. The apparatus of claim 1 further comprising separate temperaturesensing means for sensing the temperature of said mold and the moltenmetal in said crucible, signal means connected with said moldtemperature sensing means for signalling that a predetermined desiredtemperature has been achieved in said mold,

means for activating said additional heating means, and electricalcontrol means responsive to a signal from said molten metal temperaturesensing means that a predetermined desired temperature has been achievedin such molten metal automatically operative thereon to activate saidmeans for subjecting such molten metal to sufficient pressuredifferential to force such molten metal through said crucible openinginto said mold as aforesaid.

12. The apparatus of claim 1 wherein said insulated enclosure has anupper wall with an opening therein for receipt of said crucible, andmeans are provided for clamping said crucible in place within saidopening.

13. The apparatus of claim 12 further comprising a removable coverenclosing said crucible, means for admitting an inert gas to said cover,and a vent for said enclosure.

14. The apparatus of claim 12 further comprising a verticallyreciprocable support for said mold within said enclosure, and means forreciprocating said support to clamp said mold against the underside ofsaid crucible with said opening in said crucible in juxtaposition to asprue inlet in said mold.

15. The apparatus of claim 14 wherein said means for subjecting suchmolten metal to sufficient pressure differential to force such moltenmetal through said crucible opening into said mold comprises a vacuumpump, a suction passage through said support leading to said mold forapplying suction to said mold to draw such molten metal from saidcricible rapidly into said mold, and means for connecting said suctionpump to said suction passage.

16. The apparatus of claim 15 wherein said support has a platform on theupper end thereof for supporting said mold thereon, and there is asealing ring interposed between said platform and mold around the outerperiphery of said mold which serves to space the underside of said moldslightly from the upper surface of said platform to permit suction to beapplied to the entire undersurface of said mold.

17. The apparatus of claim 15 further comprising a cover over said mold,a sealing ring interposed between said cover and mold to provide anair-tight seal therebetween, said cover having a central aperturetherein in alignment with said opening in said crucible and a sprueinlet to said mold, and a sealing ring interposed between said cover andthe underside of said crucible providing an air-tight seal between saidcrucible and cover around said central aperture.

18. The apparatus of claim 1 wherein said temperature sensing meanselectrically senses the temperature of the molten metal charge in saidcrucible.

19. The apparatus of claim 1 wherein said temperature sensing meanscomprises an infrared radiation detector which is focused upon the metalcharge in said crucible to sense the temperature of such metal charge byresponse to the infrared radiation of the metal.

20. The apparatus of claim 1 wherein said additional heating meanscomprises means for passing an electric heating current through themetal charge within said crucible to cause resistance heating of thelatter to melt such metal charge.

21. The apparatus of claim 20 wherein said mold is a suction mold, andsaid means for applying such pressure differential to force such moltenmetal through said opening in said crucible into said mold comprises asuction pump for applying suction to said mold to draw such molten metalfrom said crucible into said mold.

22. The apparatus of claim 20 wherein the surface tension of such moltenmetal is sufficient to prevent passage of such molten metal through saidcrucible opening under the influence of gravity, and such pressuredifferential is effective to overcome such surface tension when applied.

1. Metal casting apparatus comprising an insulated enclosure, a moldwithin said enclosure, heating means for heating the interior of saidenclosure to heat said mold to optimum casting temperature, ametal-heating crucible above said mold having an opening incommunication with said mold within said enclosure, additional heatingmeans for rapidly heating a metal charge within said crucible to meltthe same and bring the molten metal to optimum casting temperaturewithin said crucible, said opening in said crucible being too small topermit ready passage of such molten metal therethrough, means forsensing the temperature of the molten metal in said crucible, and meansfor subjecting such molten metal in said crucible to sufficient pressuredifferential to force such molten metal through said opening into saidmold after the molten metal has reached such optimum castingtemperature.
 2. The apparatus of claim 1 wherein said additional heatingmeans for rapidly heating the metal charge in said crucible compriseselectrical contact plates connected to said crucible for passage ofelectric current through said crucible and through the metal charge tocause resistance heating of the latter.
 3. The apparatus of claim 1wherein said crucible is vertically split to provide split portions ofelectrically conductive material, with insulating material separatingsaid split portions, and said additional heating means for rapidlyheating the metal charge in said crucible comprises means for passing anelectric heating current between said split portions through the metalcharge contained therein to melt such metal charge.
 4. The apparatus ofclaim 3 further comprising means for pressing such metal charge againstsaid crucible split portions to enhance passage of such electricalcurrent through such metal charge rapidly to heat the same.
 5. Theapparatus of claim 3 further comprising a holddown member mounted tobear downwardly upon the metal charge in said crucible to insure goodelectrical contact of such metal charge with both of said crucible splitportions to enhance passage of such electric current through such metalcharge rapidly to heat the same.
 6. The apparatus of claim 5 whereinsaid hold-down member is also electrically conductive and is connectedto an electric current source for passage of current between saidhold-down member and said crucible split portions through such metalcharge.
 7. The apparatus of claim 5 wherein said hold-down member isresiliently mounted for downward pressure upon such metal charge, andstop means are provided for limiting downward movement of said hold-downmember to prevent direct contact of said hold-down member with saidcrucible split portions.
 8. The apparatus of claim 1 wherein said moldis a suction mold, and said means for applying such pressuredifferential to force such molten metal through said opening in saidcrucible into said mold comprises a suction pump for applying suction tosaid mold to draw such molten metal from said crucible into said mold.9. The apparatus of claim 8 further comprising clamping means forclamping said mold in sealing engagement with the underside of saidcrucible thereby to permit application of such suction to said cruciblethrough said mold.
 10. The apparatus of claim 1 wherein the surfacetension of such molten metal is sufficient to prevent passage of suchmolten metal through said crucible opening under the influence ofgravity, and such pressure differential is effective to overcome suchsurface tension when applied.
 11. The apparatus of claim 1 furthercomprising separate temperature sensing means for sensing thetemperature of said mold and the molten meTal in said crucible, signalmeans connected with said mold temperature sensing means for signallingthat a predetermined desired temperature has been achieved in said mold,means for activating said additional heating means, and electricalcontrol means responsive to a signal from said molten metal temperaturesensing means that a predetermined desired temperature has been achievedin such molten metal automatically operative thereon to activate saidmeans for subjecting such molten metal to sufficient pressuredifferential to force such molten metal through said crucible openinginto said mold as aforesaid.
 12. The apparatus of claim 1 wherein saidinsulated enclosure has an upper wall with an opening therein forreceipt of said crucible, and means are provided for clamping saidcrucible in place within said opening.
 13. The apparatus of claim 12further comprising a removable cover enclosing said crucible, means foradmitting an inert gas to said cover, and a vent for said enclosure. 14.The apparatus of claim 12 further comprising a vertically reciprocablesupport for said mold within said enclosure, and means for reciprocatingsaid support to clamp said mold against the underside of said cruciblewith said opening in said crucible in juxtaposition to a sprue inlet insaid mold.
 15. The apparatus of claim 14 wherein said means forsubjecting such molten metal to sufficient pressure differential toforce such molten metal through said crucible opening into said moldcomprises a vacuum pump, a suction passage through said support leadingto said mold for applying suction to said mold to draw such molten metalfrom said cricible rapidly into said mold, and means for connecting saidsuction pump to said suction passage.
 16. The apparatus of claim 15wherein said support has a platform on the upper end thereof forsupporting said mold thereon, and there is a sealing ring interposedbetween said platform and mold around the outer periphery of said moldwhich serves to space the underside of said mold slightly from the uppersurface of said platform to permit suction to be applied to the entireundersurface of said mold.
 17. The apparatus of claim 15 furthercomprising a cover over said mold, a sealing ring interposed betweensaid cover and mold to provide an air-tight seal therebetween, saidcover having a central aperture therein in alignment with said openingin said crucible and a sprue inlet to said mold, and a sealing ringinterposed between said cover and the underside of said crucibleproviding an air-tight seal between said crucible and cover around saidcentral aperture.
 18. The apparatus of claim 1 wherein said temperaturesensing means electrically senses the temperature of the molten metalcharge in said crucible.
 19. The apparatus of claim 1 wherein saidtemperature sensing means comprises an infrared radiation detector whichis focused upon the metal charge in said crucible to sense thetemperature of such metal charge by response to the infrared radiationof the metal.
 20. The apparatus of claim 1 wherein said additionalheating means comprises means for passing an electric heating currentthrough the metal charge within said crucible to cause resistanceheating of the latter to melt such metal charge.
 21. The apparatus ofclaim 20 wherein said mold is a suction mold, and said means forapplying such pressure differential to force such molten metal throughsaid opening in said crucible into said mold comprises a suction pumpfor applying suction to said mold to draw such molten metal from saidcrucible into said mold.
 22. The apparatus of claim 20 wherein thesurface tension of such molten metal is sufficient to prevent passage ofsuch molten metal through said crucible opening under the influence ofgravity, and such pressure differential is effective to overcome suchsurface tension when applied.